The staurolite and sillimanite isograds at the Grenville Front near Coniston, Ontario coincide with two discrete mylonite zones, MZ I and MZ II, respectively. MZ I is characterized by mylonitic fabrics and is associated with retrograde metamorphic features; MZ II is characterized by blastomylonitic fabrics and prograde metamorphic features. Garnet–biotite geothermometry yields a temperature of ~540 °C for staurolite-grade rocks and ~610 °C for sillimanite-grade rocks, and the spatial distribution of outcrops indicates an improbably high thermal gradient of at least 114 °C/km. This can be reconciled by recognizing that the mylonite zones represent structural discontinuities in a terrain otherwise characterized by a "normal" metamorphic gradient.MZ II is probably the older zone that formed at or near the peak of metamorphism during sustained heat flow and crustal thickening (orogeny). MZ I formed later as temperatures waned. The close proximity of MZ I to MZ II reflects epdisodic or continuous deformation confined by the northwestern boundary of the Greenville mobile belt. One of the unique characteristics of the Coniston area may be that late mylonitic deformation did not obliterate earlier blastomylonitic and phyllonitic features, making possible present-day distinction between the two mylonite zones.